It is proposed to continue the purification and characterization of the chemical and biological properties of nuclear deoxyribonucleases in human cells propagated in culture. The enzymes will be obtained in nuclei of Hela cells, normal human fibroblasts, and fibroblasts transformed in culture with the oncogenic virus SV40. The enzymes will be purified by ion exchange column chromatography and molecular sieving. Further purification will be attempted by isoelectric focussing. The properties of the enzymes will be determined by standard methods, and the biological role of those enzymes in DNA replication and/or repair will be studied by relating changes in enzyme activity to various physiological and pathological states, e.g. phases of the cell cycle, treatment with DNA damaging agents, etc.